Mid-infrared spectroscopy is used for a wide array of applications including environmental sensing, trace detection of hazardous materials, pharmaceutical manufacturing, and medical diagnostics. The most common mid-infrared spectroscopy is Fourier Transform Infrared (FTIR) spectroscopy. In FTIR spectroscopy, mid-infrared light from a broadband source (lamp, glow bar, etc.) is passed through a sample and onto an interferometer. The interferometer is adjusted and the Fourier transform of the transmitted light signal is used to determine the absorption spectrum. For example, the sample may contain some quantity of one or more volatile organic compounds (VOCs) that need to be detected, identified and/or measured. For trace detection applications, the sample path length is sometimes extended (e.g. long open path measurements) to provide larger absorption signals. However, the optical path length is typically limited in FTIR spectroscopy to 10-100 meters. Additionally, the absorption baseline is usually neither stable nor smooth (thereby limiting the accuracy with which you can determine small optical absorptions), and the spectral resolution if often limited to approximately 1.0 cm−1.
It is known that a high-finesse optical cavity can be used to provide a very long effective optical path length (1-10 km, typical). See, for example: A. O'Keefe and D. A. G. Deacon, “Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources”, Review of Scientific Instruments 59 (1988) 2544; A. O'Keefe, “Integrated cavity output analysis of ultra-weak absorption”, Chemical Physics Letters 293 (1998) 331; D. S. Baer, J. B. Paul, M. Gupta, A. O'Keefe, “Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy”, Applied Physics B 75 (2002) 261; and U.S. Pat. No. 6,795,190, “Absorption spectroscopy instrument with off-axis light insertion into cavity”.
Recent developments in quantum cascade laser (QCL) technology, optical parametric oscillators (OPOs), and difference-frequency generation (DFG) sources have resulted in widely-tunable lasers in the mid-infrared with central wavelengths spanning from 3-11 microns. Quantum cascade lasers are generally made for operation in continuous mode. Additionally, while they are tunable they are also external cavity devices with an external grating that needs to mechanically settle after tuning to a specified wavelength before stable use at that new wavelength becomes possible, particularly if accurate measurements are to be made.